loOcT., I9I2-J '/'//f Clifiinstry of Lime. 605 



continuously by removing the burnt lime at the base of the kiln, replacing 

 with a fresh supply of carlx)nate and fuel at the top Natural deposits 

 of limestone, &c., are never pure, but usually contaminated with varying 

 proportions of carbonate of magnesia, iron and alumina and silica. The 

 higher the percentage of these in the deposit, the less pure will be the 

 lime produced on burning. 



The Quality of Lime. 



This depends upon several circumstances ; impurities, as just men- 

 tioned, in the limestone, affect the quality. But quality may also be 

 affected in other ways. If the lime is not thoroughly burned, pieces of 

 imperfectly burned stone, which will not slake, will be contained in it. 

 With limestone containing much silicious matter (sand), over-burning may 

 take place, and calcium silicate be formed. Again, if some time has 

 elapsed since burning, the lime will deteriorate owing to partial slaking, 

 and subsequent formation of calcium carbonate as explained above. The 

 essential points in connexion with lime burning is that limestone of good 

 quality should be used, combined w^ith careful burning. Well burned 

 lime should contain practically all its lime as calcium oxide (CaO). 



Slaked Lime (C'alcium Hydrate, ("aH._,0;a). 



Slaked lime is produced when freshly burned limestone is treated with 

 water. The material will swell up considerably, and give off steam, 

 owing to the heat generated by the chemical combination of the lime and 

 water, thus. CaO + HoO = CaHjO.^. Eventually the mass will crumble 

 into a fine powder. 



Air Slaked Lime. 



This material differs from the water-slaked lime, in that it is a mix- 

 ture of slaked Ume and calcium carbonate formed by reason of the fact 

 that air contains both moisture and carbonic acid gas, both of which have 

 a combining affinity with the burnt lime. The relative proportion of 

 each constituent of air-slaked lime varies according to the time of ex- 

 posure to the atmosphere after burning, for, as already mentioned, lime, 

 if left long enough exposed, will eventually all become calcium carbonate 

 again. Tliis is the form of lime usually su])i)lied as agricultural lime. 



Carbonate of Lime. 



This is present in agricultural lime, having been formed from slaked" 

 lime on exposure. Carbonate of lime is also obtained in chalk and all 

 forms of limestone direct. If applied as a land dressing this form cf 

 lime, to be of the best .service, must be in as fine .state of division as pos- 

 sible, and, therefore, when purchasing limestone for such use it should 

 be stipulated for, that it be finely gicund. When finely ground, it can 

 be more evenly distributed. Its effectiveness is also increased, owing in 

 a great measure to the greater surface exposed by the fineness of rhe 

 particles to the action of the soil acids. Some deposits of carbonate of 

 lime are better adapted on that account for dressing soil than others. 

 Chalk will easily crumble into a fine powder when dug out and allowed" 

 to become dry with exposure. Other forms which bear a striking re- 

 semblance to ordinary sandstone are also easily ground. Shells, both- 

 of fresh and salt water origin, are much denser in structure, and offer 



